1. Field of the Invention
This invention relates to a disc such as an optical disc, an optical magnetic disc, a phase change optical disc and so on having an information recording area for data near a center hole of the disc and a disc manufacturing apparatus and method thereof.
2. Description of Related Art
These days, as recording media for recording digital data such as sounds, images, pictures and computer data, an optical disc becomes the mainstream. An optical disc has various size according to recording media. For example, as the representative of a disc for sound recording media, there is a compact disc whose diameter is 120 .+-.0.3 mm or 80.+-.0.3 mm, and as recording media for images and computer data and so on, there is a CD-ROM whose diameter is 120.+-.0.3 mm.
Manufacturing apparatus for producing an optical disc of this kind is described hereafter referring to FIG. 15 to FIG. 18.
FIG. 16 shows molds and a stamper 201 for manufacturing an optical disc (a compact disc whose diameter is 120 mm). FIG. 17 shows a replica disc 203 manufactured by the molds and the stamper 201 of FIG. 16. Data are recorded on the stamper 201 from the inside diameter 46 mm to 117 mm. The replica disc 203 becomes an optical disc as a final product. A fixed mold 210 has a mirror block 211 having a mirror surface 213. The stamper 201 is fixed on the mirror surface 213 by an outer stamper holder 215 and an inner stamper holder 217. A moving mold 220 has a mirror block 221 having a mirror surface 223. The mirror block 221 contains a sprue bush 225 of pipe shape. An injection port 227 for injecting a forming material melted by heat is formed at the sprue bush 225 and its top end is exposed from the mirror surface 223.
The fixed mold 210 contains a cut pin 231 of pipe shape. A sprue ejector 233 of bar shape is disposed at the center of the cut pin 231. A top end of the sprue ejector 233 caves in a little from a top end of the cut pin 231 and forms a concave shape, so that a space 235 is provided there. The sprue bush 225, cut pin 231 and sprue ejector 233 can move in the direction of an arrow 241. The sprue bush 225, cut pin 231 and sprue ejector 233 define a center hole 205 as a through hole having an inside diameter of 15 mm to 15.1 mm on the replica disc 203, in case of a compact disc. A sprue 207 is taken out from the replica disc 203 by the sprue ejector 233.
The process to manufacture an optical disc by using the above optical disc manufacturing apparatus is described hereafter. First of all, as shown in FIG. 16, in the state that the fixed mold 210 and the moving mold 220 are opened, the stamper 201 is fixed on the mirror surface 213 of the mirror block 211 of the fixed mold 210 by the holders 215 and 217 so that a data transfer surface, as an information recording area, of the stamper 201 is outside. Then, the moving mold 220 moves toward the fixed mold 210 and both molds 210 and 220 are unified by closing them each other. Next, as shown in FIG. 15, a material melted by heat, for example, synthetic resin such as polycarbonate and so on, are injected from the injection port 227 into a disc molding space defined by the data transfer surface of the stamper 201 on the fixed mold 210 and the mirror surface 223 of the mirror block 221 of the moving mold 220. The shape of the replica disc 203 at this time is shown in FIG. 17(a). The material fill up the molding space and injection pressure of the materials at this time transfers the data recorded on the stamper 201 to a boundary surface of the material between the stamper 201 and the material. Then, the center hole 205 is formed by moving the sprue bush 225, cut pin 231 and sprue ejector 233 in the direction of an arrow 243 before the melting material cool down, as shown in 17(c). At this time, the intervals among the sprue bush 225, cut pin 231 and sprue ejector 233 are unchanged. Next, the material filling the molding space is cooled to be solidified.
Thereafter, the moving mold 220 moves to separate from the fixed mold 210 and the molds 210 and 220 are opened. At the same time, the sprue bush 225 moves to the original position in the direction of an arrow 245 so that the top of the sprue bush 225 reaches the same level as the mirror surface 223. Next, the sprue ejector 233 moves further in the direction of the arrow 243 and a sprue 237 is taken out from the replica disc 203, as shown in 17(d). The sprue 237 is the material filled at the injection port 227 and the center hole 205 at manufacturing time for a disc and it is unnecessary. After taking out the sprue 237, the cut pin 231 and the sprue ejector 233 move back to the original position in the direction of the arrow 245. By the above-described process of manufacturing optional discs, the data recorded on the stamper 201 is transferred to the replica disc 203, and such a replica disc 203 having the center hole 205 at its center is taken out. Then, an optical disc having a center hole is manufactured by providing a reflective film and the like on the information recording surface thereof.
An example of an optical disc manufactured by the above process is shown in FIG. 18.
FIG. 18 is a plan view of a compact disc manufactured by a common injection mold method.
As shown in FIG. 18, a mold used for mass production bores a center hole 252, whose inside diameter is from 15 mm to 15.1 mm (in case of a compact disc) prescribed by International Electrotechnique Commission (IEC), through an information recording area 251, shown by cross-hatching, of an optical disc body 250 during manufacturing process. A recording start point 253 of the information recording area is at an inside diameter of 46 mm.
In the above disc manufacturing apparatus, boring operation for the center hole is performed during the disc molding process, so that manufacturing steps increases. In addition, the material filled in a center part of a disc, namely, a sprue part, are wasteful. Moreover, compaction of a disc is demanded as amount of information is expanded due to diversification of multi-media. That is, a disc whose diameter or size is made smaller than the present one will be needed. As an example of a compact disc on the market, a layout on a plane of a well-known compact disc shown in FIG. 18 is as follows:
the outside diameter=120 mm PA1 the largest diameter of the information recording area (recording end point)=117 mm PA1 the smallest diameter of the information recording area (recording start point)=46 mm PA1 center hole (through hole)=15 mm
Here, though the outside diameter of a disc is 120 mm, the largest diameter of the information recording area, namely, the position of a lead-out 254 (recording end point) is 117 mm, and the smallest diameter of the information recording area, namely, the position of a lead-in 253 (recording start point) is 46 mm. Therefore, only 80 percent of the whole disc area is used as an information recording area. In addition, in case of a compact disc and a minidisc which have a smaller diameter, the utility as an information recording area is lowered further.